all: Remove symlink support on Windows, SymlinksEnabled config

After this change,

- Symlinks on Windows are always unsupported. Sorry.

- Symlinks are always enabled on other platforms. They are just a small
  file like anything else. There is no need to special case them. If you
  don't want to sync some symlinks, ignore them.

- The protocol doesn't differentiate between different "types" of
  symlinks. If that distinction ever does become relevant the individual
  devices can figure it out by looking at the destination when they
  create the link.

It's backwards compatible in that all the old symlink types are still
understood to be symlinks, and the new SYMLINK type is equivalent to the
old SYMLINK_UNKNOWN which was always a valid way to do it.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3962
LGTM: AudriusButkevicius